If you just want to disallow the entire RM, then you can block /resources/. However, if you are trying to block a single category, you can't do this via robots.txt. You would need to modify the resource_view template:
Code:
<xen:if is="{$resource.resource_category_id} == 123">
<xen:container var="$head.noindex"><meta name="robots" content="noindex" /></xen:container>
</xen:if>
(I haven't tested this, so please check the code is being output when you expect.)